Question: $ A = \left[\begin{array}{rr}4 & 1 \\ 4 & -1 \\ 4 & 1\end{array}\right]$ What are the dimensions of matrix $ A$ ?
Answer: The first dimension is the number of rows in the matrix. How many rows does $ A$ have? The second dimension is the number of columns in the matrix. How many columns does $ A$ have? So, $ A$ is a 3 $\times$ 2 matrix.